For presentations, I don't use PowerPoint, mostly because I think it encourages a very shallow and simplistic way of thinking and presenting ideas, where the focus is mostly on visual appearance and easy-to-read bullet lists, rather than on a deeper level of structured and sophisticated thinking. This article by Edward Tufte argues quite nicely for this view; and this page cites a recent study that shows what I always suspected: that the use of animations in presentations has a negative effect on comprehension.
What I do use is the LaTeX package called Beamer. Unlike PowerPoint, which encourages you to focus on the visual aspects of your presentation and to construct a linear list of slides with silly transitions, Beamer (like LaTeX in general) encourages you to focus on what really matters: the presentation's logical structure and content. Additionally, you get the usual advantages of LaTeX, such as support for numbered linguistic examples, cross-references, trees etc.
To make things even easier, I use org-mode in Emacs to draft and/or edit my beamer presentations (see also my general notes on org-mode here).
Here's a brief guide to using Beamer, from the LaTeX for Logicians web site.
Another similar tool worth checking is powerdot.
